The present invention relates to a pressure sensor and a method of manufacturing the same and, more particularly, to a pressure sensor using a piezoelectric or capacitive semiconductor sensor chip and a method of manufacturing the same.
In conventional packaging of a pressure sensor of this type, a sensor chip made of a semiconductor is generally mounted on a metal base (to be referred to as a metal stem hereinafter). In this case, a glass buffer member is conventionally formed between the metal stem and sensor chip in order to prevent the sensor chip from being broken by a stress generated during heating due to a difference in coefficient of thermal expansion between the metal stem and sensor chip.
Electrode pins for outputting electrical signals are fixed in a plurality of through holes formed in the metal stem around the sensor chip with low-melting glass to be hermetically sealed and insulated. The electrodes of the sensor chip and the electrode pins are connected to each other through bonding wires.
A conventional pressure sensor shown in FIG. 12 is constituted by a sensor chip 101, bonding wires 102, electrode pins 103, a buffer member 104, hermetic seal glass members 105, a metal stem 106, a metal container 110, and a barrier diaphragm 111. The metal container 110 and barrier diaphragm 111 are indicated by alternate long and two short dashed lines to show the internal structure of the pressure sensor.
The sensor chip 101 is a piezoelectric or capacitive pressure sensor chip made of a semiconductor such as silicon. The bonding wires 102 electrically connect the electrode pads (not shown) of the sensor chip 101 and the electrode pins 103 to each other. The electrode pins 103 are made of a conductive metal and are fixed in through holes 106a (FIG. 13) formed in the metal stem 106 to extend through them.
The buffer member 104 formed on the metal stem 106 is made of a glass material to prevent the sensor chip 101 from being broken by a difference in coefficient of thermal expansion between the sensor chip 101 and metal stem 106. The hermetic seal glass members 105 are made of a low-melting glass, and realize hermetic seal in the gaps between the inner surfaces of the through holes 106a and the electrode pins 103.
The metal stem 106 has a disk-like shape, and five through holes 106a are formed in it to correspond to the electrode pins 103. The metal container 110 has a cylindrical shape to cover the whole sensor chip 101 mounted on the metal stem 106. The interior of the metal container 110 is filled with sealed oil such as silicone oil. The barrier diaphragm 111 is comprised of a flexible metal film and deforms in accordance with an external pressure to transmit it to the sensor chip 101 through the silicone oil. When the external pressure disappears, the barrier diaphragm 111 is restored to the initial position.
As shown in FIG. 13, the buffer member 104 is formed at the central portion of the metal stem 106, and the sensor chip 101 is placed on the buffer member 104. The sensor chip 101 and electrode pins 103 are connected to each other through the bonding wires 102.
The through holes 106a formed in the metal stem 106 have diameters slightly larger than the diameters of the electrode pins 103. The hermetic seal glass members 105 are interposed in the gaps between the electrode pins 103 and the inner surfaces of the through holes 106a. The hermetic seal glass members 105 are softened once by heating, and then cooled to solidify. The gaps between the metal stem 106 and electrode pins 103 are completely hermetically sealed. Therefore, the silicone oil filled in the metal container 110 will not leak to the lower side of the metal stem 106 through the through holes 106a. 
A method of manufacturing the conventional pressure sensor described above will be described with reference to FIG. 14. As shown in FIG. 14, the electrode pins 103 are inserted in holes 105a of the cylindrical hermetic seal glass members 105. The hermetic seal glass members 105 with the inserted electrode pins 103 are inserted in the through holes 106a of the metal stem 106, and the metal stem 106 is placed on a jig 107 having holes 107a. The holes 107a have diameters substantially equal to the diameters of the electrode pins 103. The electrode pins 103 projecting from the hermetic seal glass members 105 are inserted in the holes 107a. 
In this manner, the metal stem 106 that supports the electrode pins 103 is placed on the jig 107, and the metal stem 106 is heated until the hermetic seal glass members 105 are softened. The metal stem 106 is then cooled to solidify the softened hermetic seal glass members 105, thereby hermetically sealing the gaps between the electrode pins 103 and through holes 106a completely. A base unit 112 is thus completed.
Meanwhile, a wafer comprised of sensor chips and a glass plate are connected to each other by anodic bonding, and the resultant structure is diced, thereby forming a plurality of sensor chips 101 with buffer members 104 bonded to their lower surfaces. The sensor chip 101 is then adhered onto the base unit 112 through the buffer member 104, and electrode pads (not shown) on the sensor chip 101 and the electrode pins 103 are connected to each other through the bonding wires 102. The whole sensor chip 101 is covered by the metal container 110 and barrier diaphragm 111, and silicone oil is injected into the metal container 110, thus completing a pressure sensor.
FIG. 15 shows a state wherein the electrode pins 103 are positioned by the jig 107. As shown in FIG. 15, since the diameters of the holes 107a of the jig 107 are substantially equal to the diameters of the electrode pins 103, the electrode pins 103 are positioned at the centers of the corresponding through holes 106a by the jig 107. Even when the hermetic seal glass members 105 are softened by heating, the positions of the electrode pins 103 will not shift.
The conventional pressure sensor described above has the following problems.
As described above, to fabricate a pressure sensor structure shown in FIG. 12, at least three bonding steps are required, i.e., the first step of hermetically sealing the metal stem and electrode pins with glass, the second step of bonding (bonding a wafer before dicing) the sensor chip and glass buffer member by anodic bonding, and the third step of adhering the glass buffer member and metal stem.
Too many steps are necessary in this manner, leading to a possible decrease in quality and increase in cost.
When a diaphragm structure using the barrier diaphragm 111 and the sealed oil is employed, since the amount of sealed oil is large, the temperature characteristics of the pressure sensor degrade. For this reason, conventionally, a structure made of glass, a ceramic material, a resin, or the like is formed around the buffer member 104 to decrease the amount of sealed oil. This, however, increases the number of steps and the number of components in turn.
In adhesion of the buffer member 104 and metal stem 106, glass sealing, and die bonding such as fixing, stress-free bonding cannot be achieved unless not only the coefficients of thermal expansion of the buffer member 104 and metal stem 106 are matched but also the coefficient of thermal expansion of the adhesive that adheres the buffer member 104 and metal stem 106 is also matched with those of the buffer member 104 and metal stem 106. In order to avoid this stress, conventionally, the thickness of the buffer member 104 is increased, so that the amount of stress propagating to the sensor chip 101 is attenuated. As the size of the buffer member 104 increases, however, the amount of sealed oil increases, and dicing of the buffer member 104 becomes difficult, which are new problems.
Although the metal stem 106 and electrode pins 103 preferably abut against each other in order to improve the pressure resistance against an external pressure, electrical insulation cannot be maintained between them since they are made of a metal. Therefore, conventionally, the thickness of the metal stem 106 is increased and the lengths of the hermetic seal glass members 105 of the metal stem 106 are increased. When, however, the through holes 106a formed in the metal stem 106 become long, they are difficult to form simultaneously by pressing, and must be formed one by one by cutting.
Due to limitations on the heat-resistant temperatures and heating temperatures of the respective steps, the temperature for the die bonding step must match the lowest temperature, and accordingly the reliability at the bonding portion decreases.
Since a decrease in insulating distance of the hermetic seal glass members 105 and a decrease in distance of wire bonding are limited, it is difficult to fabricate a multi-pin structure.
The electrode pins 103 must be accurately arranged at the central portions of the through holes 106a from the viewpoint of dielectric breakdown. For this purpose, the positioning precision of the electrode pins 103 must be high, leading to a requirement for a high size precision of the metal stem 106 and jig 107. Since the electrode pins 103 and jig 107 must be fitted with each other with a high fitting precision, a precision concerning the bend of the electrode pins 103 or the like becomes strict.
As shown in FIG. 16A, if the hole 107a of the jig 107 and the hole 105a of the hermetic seal glass member 105 shift from each other, the electrode pin 103 cannot be inserted in the hole 107a of the jig 107. For this reason, the hole 107a of the jig 107 and the hole 105a of the hermetic seal glass member 105 must be aligned with each other accurately. However, the larger the number of electrode pins 103, the more difficult alignment between the jig 107 and metal stem 106.
Due to heating, hermetic seal glass might run off, and the jig 107 and metal stem 106 may be undesirably adhered to each other.
If the high pressure resistance of the metal stem 106 is to be maintained and the shapes of the holes formed by cutting and pressing are to be maintained, the distance among the through holes 106a of the metal stem 106 cannot be decreased to be smaller than a predetermined distance, and the package cannot be downsized.
To prevent oxidation of and to remove oxide films from the surfaces of the electrode pins 103 and metal stem 106, heating may be performed in an oxygen-free atmosphere gas, e.g., a reducing atmosphere of a hydrogen-containing nitrogen or argon atmosphere (sometimes a 100%-hydrogen atmosphere). By this heating, glass hermetic sealing and brazing can be performed simultaneously. However, when an alkali metal oxide added to decrease the softening point (and the pour point) of the hermetic seal glass members 105 is reduced by hydrogen, an alkali metal is deposited to sometimes impair electric insulation and decrease the breakdown voltage, cause a stray current, or degrade and break a p-n junction.
When the hermetic seal glass member 105 runs off due to heating, it forms an unnecessary fillet shape. When a component for decreasing the amount of sealed oil is to be built in the structure, the fillet shape interferes with it to make it difficult to built the component into the structure. In order to avoid this problem, if the heating time and temperature are controlled so the hermetic seal glass member 105 will not run off, the dielectric breakdown, pressure resistance, and seal performance may undesirably degrade.
When the softened hermetic seal glass member 105 runs off, if its amount has not been precisely adjusted, the hermetic seal glass member 105 that has been squeezed out solidifies as it attaches to the electrode pin 103 or the like, this forming a fillet shape 105b. This fillet shape 105b is an obstacle when a component is to be built around the electrode pin 103.
When an unnecessary fillet shape is formed by running off of the hermetic seal glass, unless the electrode pin 103 is formed to be higher than the fillet shape, glass attaches to a bonding pad at the distal end of the electrode pin 103, thus disabling wire bonding. Therefore, the electrode pins 103 must be formed to be higher than a predetermined height, thus interfering with downsizing of the pressure sensor.
As described above, in the conventional pressure sensor, since the glass buffer member 104 and metal stem 106 are used and a jig is used for positioning the electrode pins 103, various problems arise.
It is an object of the present invention to provide a pressure sensor in which the number of bonding steps and the number of components are decreased to decrease the cost and to improve the quality, and a method of manufacturing the same.
It is another object of the present invention to provide a pressure sensor which does not require a positioning jig, and a method of manufacturing the same.
In order to achieve the above objects, according to the present invention, there is provided a pressure sensor comprising a base unit on which a sensor chip is mounted, a cap-like metal container which is fixed to the base unit so as to seal the sensor chip and in which oil is sealed, and a flexible diaphragm forming part of the metal container to transmit an external pressure by displacement thereof to the sensor chip through the oil, the base unit having a metal cylindrical member, a first positioning member fitted on an inner side of the cylindrical member and made of an insulating material, a lead member supported to extend through the first positioning member and electrically connected to the sensor chip through a bonding wire, and a hermetic seal layer formed on the first positioning member fitted in the cylindrical member to hermetically seal a portion between the cylindrical member and the first positioning member and a portion between the lead member and the first positioning member, the hermetic seal layer being made of an insulating material which is softened at a temperature lower than that of the positioning member.